1. Field of the Invention
The present invention relates to correction of light amount variations of a light source in an electrophotographic image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus performs light amount control (automatic power control; hereinafter, referred to as APC control) to stabilize the light amount of laser light (light beam) emitted from a semiconductor laser to expose a photosensitive member. For example, Japanese Patent Application Laid-Open No. 2004-223716 discusses the foregoing APC control. In an image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2004-223716, a light receiving element (photoelectric conversion element) receives laser light in each scanning cycle of the laser light. The value of a current supplied to the semiconductor laser is controlled so that the amount of light that is incident on the light receiving element coincides with a target light amount.
FIG. 7 is a chart illustrating a light emission characteristic of a semiconductor laser (relationship between a driving current and an emitted light amount). The value of the driving current for a target light amount P varies with temperature. The APC control discussed in Japanese Patent Application Laid-Open No. 2004-223716 can be performed in each scanning cycle or a plurality of scanning cycles to control the laser light to the target light amount even if the temperature of the semiconductor laser varies due to the emission of the laser light.
If a constant current continues to be supplied to a semiconductor laser for a relatively long period, the semiconductor laser itself causes self-heating. Some semiconductor lasers have a characteristic that the light amount of the laser light is attenuated due to self-heating after a start of the supply of the driving current and stabilizes after a lapse of certain time. Such a characteristic is commonly referred to as a droop characteristic of the semiconductor lasers. Some semiconductor lasers have a characteristic that the light amount of the laser light gradually increases due to self-heating after a start of the supply of the driving current and stabilizes after a lapse of certain time. Such a characteristic is commonly referred to as an inverse droop characteristic of the semiconductor lasers. Variations in the light amount of the laser light can cause uneven potentials of exposed portions formed on the photosensitive member, and as a result, the amount of toner adhering to the exposed portions becomes uneven. This leads to a drop in the image quality of the output image.
To correct such a droop characteristic or inverse droop characteristic, Japanese Patent Application Laid-Open No. 2012-40779 discusses an image forming apparatus that stores predetermined correction data in a memory in advance and corrects the driving current supplied to the semiconductor laser by using the correction data.
As illustrated in FIG. 8, the droop characteristic or inverse droop characteristic varies with temperature. More specifically, the amount of attenuation or the amount of gradual increase per unit time varies with the temperature of the semiconductor laser. The image forming apparatus that corrects the driving current supplied to the semiconductor laser by using the same correction data regardless of the temperature of the semiconductor laser therefore may fail to sufficiently correct the droop characteristic or the inverse droop characteristic, depending on the temperature of the semiconductor laser.